1. Field of the Invention
This invention relates to an optical mount, and more particularly, to an optical mount used for adjusting the direction of a laser beam generated by a laser diode module and used, within a sensor, for detecting particulate matter in a stream of air or water.
2. Prior Art
Many types of optical mounts have been designed which provide convenience in adjusting the position and/or orientation of various types of individual optical components or of entire optical systems. A wide variety of general purpose optical mounts for lenses, mirrors, lasers, fiber optics, and the like are commercially available. However, these generic mounts are typically useful only for laboratory "breadboard" or proof-of-concept experiments and, regardless of manufacturers' claims to the contrary, are usually not suitable for incorporation into actual deliverable hardware. Optical tracking systems for missiles and laser-based scanners for point-of-sale terminals, for example, almost always incorporate mounts that are unique to a particular set of optical and mechanical requirements.
Similarly, particle sensors based upon the use of laser technology fall into the category of electro-optical equipment which, in the prior art, has been mounted using hardware designed to meet a specific set of requirements. In such a particle sensor, an optical system is used to collect the light from a laser source and to re-direct the laser light, as a beam, into a small sensing volume which intersects a portion of the stream of particulate matter. Particle sensors having a detection system that measures the degree of light extinction utilize the fact that small particles which pass through the sensing volume scatter light away from the main beam, thus reducing the beam intensity. Alternately, a particle sensor can be based on a detection system which measures the extent of light scattering. In the latter sensor, high numerical aperture optics are used to collect the light scattered by the small particles which pass through the sensing volume. The scattered light pulses are brought to focus on a sensitive detector.
In the prior art, mounts for a laser and for a beam-forming optical assembly, or lens, in a particle sensor are considered to have separate functions and are separated physically. Typically, the laser is adjustable in position within a plane disposed generally perpendicularly to the direction of the laser beam. The beam-forming optics, on the other hand, are distributed along an axis disposed perpendicularly to said plane and can be moved only with respect to this axis. So moving the beam-forming optics allows the focal point of the laser beam to be adjusted.
To a first approximation, adjustments in the position of the laser produce the desired beam steering effect; and adjustments in the position of the beam-forming optics provide for variations in laser beam focus. In practice, however, one quickly discovers that, while these adjustments are individually easy to implement, they must be performed in succession and iteratively many times to overcome important higher order effects.
These higher order effects are caused by a complex interaction between the laser beam and the beam-forming optics. If the beam-forming optics have a large conjugate ratio, small changes in the position of the laser beam at the object plane can result in large changes in the position of the beam at the image plane inside the sensing volume. Moreover, shifts in the position of the laser in a direction generally perpendicularly to the original direction of the laser beam cause the distance between the laser and the beam-forming optics to change. As a result, the focal characteristics of the laser beam within the sensing volume are affected. In order to compensate for this effect, the position of the beam-forming optics must be adjusted to restore focus. This adjustment, in turn, has a small, but perhaps measurable, effect on the alignment of the laser beam within the sensing volume.
An additional complication arises when the laser beam is sufficiently far out of alignment with the optical axis of the beam-forming optics so as to affect the beam quality.
In general, the sensitivity and resolution of a laser-based particle sensor can be seriously degraded by slight misalignments in its optical system. Such misalignments can be caused by shock, vibration, and/or temperature changes. Preventing a shock or vibration-induced misalignment requires that certain components of the optical system, once positioned, be rigidly retained in place. At the same time, mounts employed to so retain these components must be robust in construction and adjustable with a high degree of sensitivity, at least at the outset, in order to achieve peak performance from the sensor.
It is well known to persons skilled in the prior art that the alignment procedures for laser-based particle sensors are iterative, difficult, and time-consuming. A typical alignment can take an operator an hour to accomplish.